Noise canceling microphones (referred to interchangeably herein as “NC mics”) are a desirable option for communication headsets used in potentially noisy environments. Unlike simpler omindirectional microphones (referred to interchangeably herein as “omni mics”), a NC mic has reduced sensitivity to distant sounds. Instead, a NC mic is generally more sensitive where the distance from the NC microphone to the acoustic source is located nearby, as opposed to sources of ambient or background sounds. Often times, however, a user does not optimally position a NC mic, like that included in a communications headset. The mispositioned NC microphone reduces sensitivity to the user's voice, and this in turn reduces the signal-to-noise ratio (SNR). Severe misposition of the NC mic is also problematic because the user's voice will be attenuated to a degree that is unintelligible for the receiving entity (e.g., person or machine) on the other end of the communication link.
This deleterious effect is caused by two related characteristics of NC mics. A first order NC mic measures the sound pressure at two nearby points in space. This can be accomplished via a single mic with sound ported to the front and rear of a diaphragm, typically from two openings acting as pick-up points for the single mic. The sound waves received at the two openings impinge on both sides of the diaphragm so that noise canceling effects may be effectuated. A NC mic measuring sound pressure at two nearby points in space may also be constructed from two separate mics electrically connected, each with a single opening for sound to reach the diaphragm. To form a NC mic assembly from the two separate mics, the near instantaneous difference of the sound pressure level at the two points is taken. This difference typically gives the NC mic assembly a polar pattern (i.e., different sensitivity from different directions) and a proximity effect (i.e., greater drop-off with increasing distance than one omni mic). Whichever way the NC mic is formed, the problem remains that either angular mispositioning (i.e., polar pattern related) or distance mispositioning (i.e., proximity effect related) or the combination of the two lead to degraded SNR.
One conventional technique for preventing a microphone from becoming mispositioned with respect to an acoustic source entails headsets integrated with the microphone, where a cup is provided to align the user's chin to the mic. This technique is cumbersome and impractical for lightweight communications headsets, which generally require the user to readily and quickly put on and take off the headset.
Another conventional technique used to accommodate the mispositioning of a microphone includes the lowering of the microphone's noise canceling effectiveness as a tradeoff for more flexibility of the microphone positioning. This technique is also unsatisfactory because it effectively causes the microphone to be less useful in high noise environment applications. Additionally, this conventional technique also comprises the headset having less noise canceling features.
Yet another conventional technique that attempts to address the mispositioning of a microphone utilizes a nonlinear Automatic Gain Control (AGC) amplifier. The AGC amplifier typically provides low gain when the microphone output is low and provides high gain when the microphone output is high. This technique is problematic because it does not increase the SNR when the user is speaking, but rather reduces background noise during pauses in between speech.
Accordingly, what is needed is a technique for determining whether a microphone assembly, such as used in a lightweight headset, is positioned incorrectly with respect to an acoustic source, such as a user's mouth. Upon determining that a microphone assembly is mispositioned, it is desirable to have a way for the microphone assembly to provide automatic compensation of signal degradation resulting from the mispositioned microphone assembly. It is further desired to have a manner of detecting and of compensating for a situation where the acoustic signal from the acoustic source is too low. Also, it would be beneficial if the NC microphone assembly provided a user an indication of the proximity and angular error resulting from the microphone misposition, so that the microphone assembly may be repositioned.